extensions: xt_socket: add txlate support for socket match

extensions: xt_socket: add txlate support for socket match

v2: document the match semantics of -m socket.

Ignore --nowildcard if used with other options when translating
and add "wildcard 0" if the option is missing.

"-m socket" will ignore sockets bound to 0.0.0.0/:: by default,
unless --nowildcard is given.

So, xlate must always append "wildcard 0", can elide "wildcard"
if other options are present along with --nowildcard.

To emulate "-m socket --nowildcard", check for "wildcard <= 1" to
get a "socket exists" type matching.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>